ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-4557-2011 Modeling the Frozen-In Anticyclone in the 2005 Arctic Summer Stratosphere Allen D. R. 1 Douglass A. R. 2 Manney G. L. 3 4 Strahan S. E. 5 Krosschell J. C. 6 Trueblood J. V. 6 Nielsen J. E. 2 7 Pawson S. 2 Zhu Z. 2 7 Naval Research Laboratory, Washington, DC, USA NASA Goddard Space Flight Center, Greenbelt, MD, USA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA Department of Physics, New Mexico Institute of Mining and Technology, Socorro, NM, USA Goddard Earth Science and Technology Center, University of Maryland Baltimore County, Baltimore, MD, USA Dordt College, Sioux Center, IA, USA Science Systems and Applications Inc., Lanham, MD, USA 13 05 2011 11 9 4557 4576 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/11/4557/2011/acp-11-4557-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/4557/2011/acp-11-4557-2011.pdf

Immediately following the breakup of the 2005 Arctic spring stratospheric vortex, a tropical air mass, characterized by low potential vorticity (PV) and high nitrous oxide (N<sub>2</sub>O), was advected poleward and became trapped in the easterly summer polar vortex. This feature, known as a "Frozen-In Anticyclone (FrIAC)", was observed in Earth Observing System (EOS) Aura Microwave Limb Sounder (MLS) data to span the potential temperature range from ~580 to 1100 K (~25 to 40 km altitude) and to persist from late March to late August 2005. This study compares MLS N<sub>2</sub>O observations with simulations from the Global Modeling Initiative (GMI) chemistry and transport model, the GEOS-5/MERRA Replay model, and the Van Leer Icosahedral Triangular Advection (VITA) isentropic transport model to elucidate the processes involved in the lifecycle of the FrIAC, which is here divided into three distinct phases. During the "spin-up phase" (March to early April), strong poleward flow resulted in a tight isolated anticyclonic vortex at ~70–90° N, marked with elevated N<sub>2</sub>O. GMI, Replay, and VITA all reliably simulated the spin-up of the FrIAC, although the GMI and Replay peak N<sub>2</sub>O values were too low. The FrIAC became trapped in the developing summer easterly flow and circulated around the polar region during the "anticyclonic phase" (early April to the end of May). During this phase, the FrIAC crossed directly over the pole between 7 and 14 April. The VITA and Replay simulations transported the N<sub>2</sub>O anomaly intact during this crossing, in agreement with MLS, but unrealistic dispersion of the anomaly occurred in the GMI simulation due to excessive numerical mixing of the polar cap. The vortex associated with the FrIAC was apparently resistant to the weak vertical shear during the anticyclonic phase, and it thereby protected the embedded N<sub>2</sub>O anomaly from stretching. The vortex decayed in late May due to diabatic processes, leaving the N<sub>2</sub>O anomaly exposed to horizontal and vertical wind shears during the "shearing phase" (June to August). The observed lifetime of the FrIAC during this phase is consistent with timescales calculated from the ambient horizontal and vertical wind shear. Replay maintained the horizontal structure of the N<sub>2</sub>O anomaly similar to MLS well into August. Isentropic simulations using VITA also captured the horizontal structure of the FrIAC during this phase, but small-scale structures maintained by VITA are problematic and show that important mixing processes are absent from this single-level simulation.

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